National and state standards often list forms of energy that students should know, including gravitational, kinetic, potential, etc. Form can be a useful shorthand for describing the state of the system, or it can be a meaningless label to be memorized. Most physics instruction does not emphasize a deep understanding of the physical meaning of form. Are there ways that our instruction could more effectively help students gain an understanding of form? One way to develop and test understanding of forms of energy is to ask the question, "What must be considered when deciding whether a new form is legitimate?" We present case studies of students struggling with the legitimacy of forms of energy not listed in the standards, some of which they deem to be legitimate and some of which they do not. Finally, we suggest instructional methods to take advantage of this struggle.